1. Field of the Invention
This invention relates to viscous shear couplings. Such assemblies find particular, but not exclusive, use in the drive transmissions of motor vehicles.
2. Description of Prior Art
A viscous shear coupling, as described for example in United Kingdom Patent Specification No. 1,357,106 (and corresponding U.S. Pat. No. 3,760,922 issued 25th Sept. 1973) comprises a housing part, a hub part within the housing part, the parts being relatively rotatable about a common axis, and two sets of interleaved plates. The plates are planar and provided with slots and/or holes and the plates carried by the hub part alternate with those carried by the housing part. The plates of one of the sets are rotatable with the housing part and the plates of the other set are rotatable with the hub part. The plates of one of the sets are axially movable into contact with the plates of the other set. The housing part contains a viscous liquid such, for example, as a silicone oil.
The specifications also show a coupling associated with an inter-axle differential gear in a four-wheel drive vehicle.
Normally torque is transmitted between the housing part and the hub part due to shearing forces in the viscous liquid between adjacent plates, i.e. in viscous mode. The above patent specifications describe that the housing part may be filled with viscous liquid, e.g. a silicone oil, to a fill factor which leaves part of the housing normally filled with air or gas.
Such couplings exhibit a phenomenon which has come to be known as the "hump" mode in which the plates of the two sets come into contact and torque is transmitted by metallic friction which may be accompanied by shearing of the liquid between the plates.
Prolonged slipping of the coupling in "viscous mode" causes heating and expansion of the silicone oil. When the oil has expanded to fill the entire space within the coupling and all the air or gas has been dissolved in the oil there is a sharp increase in pressure in the coupling and, due to the throttling effect between adjacent plates, the movable plates of the one set are moved into contact with the plates of the other set by the fluid pressure in the coupling.
The movement of the movable plates is due to the fact that these plates do not normally remain in a central position between two fixed plates. Thus there is a wider and a narrower gap at the respective sides of a movable plate. As a result of the throttling effect, the pressure increases more rapidly in the larger gaps so that the movable plates move towards the nearer adjacent fixed plates.
When a movable plate contacts a fixed plate, frictional forces are developed so that the coupling operates in a dual mode, i.e. on the one hand the viscous liquid is sheared and on the other hand there is a frictional contact between the plates so that the coupling tends to act as a friction clutch.
This hump effect acts to protect the coupling against overloading since in this condition it is able to transmit more torque than it could in the viscous mode because of the friction clutching effect.
In the hump mode, the viscous fluid can cool down until the coupling then resumes torque transmission in the viscous mode.
If the coupling is used in a limited-slip differential gear of a motor vehicle then, if an obstacle is encountered, considerable torque can be transmitted for a short time in the hump mode to overcome the obstacle.
In the prior art couplings as described above, the plates carried by the hub part and by the housing part alternate and thus when the coupling is in the hump mode a movable plate engages on one side of a fixed plate. Due to the pressures involved, it has been found that the fixed plates may distort or "dish". This distortion may be permanent and can prevent the plates contacting one another effectively in the hump mode and due to the distortion may even prevent the movable plates moving at all.